Electrostatic deposition and electrical mobility size separation of airborne particles are widely used techniques for the collection or analysis of airborne particles. These methods require that the particles to be collected or analyzed carry an electric charge. However, for very small particles with diameters less than about 50 nm, adding an electrical charge is difficult. In this size range exposure to a bipolar ion source provides singly charged particles, but the charging efficiency is low. For particles with diameters of 50 nm, just 17% of the particles will acquire a positive charge, with an approximately equal number acquiring a negative charge. At 10 nm the fraction of particles charged with a single polarity is ˜4%, and at 3 nm this drops to less than 2%. Unipolar charging can improve charging efficiencies for particles above about 10 nm, but it also becomes ineffective at smaller particle sizes.
One technique that has been used to increase the charging efficiency of these small particles is condensation-enhanced particle charging, wherein the particles are grown through condensation, charged and re-evaporated. Some prior art techniques have used butanol condensation to prepare highly charged particles in the 10-30 nm size range. Others have used condensation of glycol to enhance the charging of sub-20 nm particles. Still others have explored this approach with water condensation, albeit for larger (80-130 nm) particles. Limitations of these existing methods are: (1) the contamination of the particle through the use of organic materials as the condensing vapor, (2) addition of multiple electrical charges to each particle, and (3) inability to charge particles below about 10 nm.